"One researcher has decided he wants to make Skype open source by reverse engineering the protocol the service uses. In fact, he claims to have already achieved that reverse engineering feat on a new skype-open-source blog. The source code has been posted for versions 1.x/3.x/4.x of Skype as well as details of the rc4 layer arithmetic encoding the service uses."

Skype is developed for a limited number of platform: windows, mac, linux, android, ios, wm7. some other platform had their support removed wm6 and many others don't have support *BSD, Sparc, Haiku, ...
Not all platform are equally developed, some are left far behind (Linux).

Having an open source Skype will give the possibility to put skype everywhere. Also it will give access to the full access to the protocol, with its full possibilities, not just what Skype/Yahoo/Microsoft want to give to the users.